1. Field of Invention
The invention described herein relates to a method of obtaining information for use in modeling a lithographic process, and a lithographic arrangement for carrying out that method. The lithographic arrangement may be, or may include, a lithographic apparatus as described below.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus are used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as, for examples, a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) having a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning-direction”) while synchronously scanning the substrate parallel or anti parallel to this direction.
When providing pattern features on a substrate, it is usual to make use of a model of a lithographic process (often referred to as a lithographic process model). A model of a lithographic process allows the processing of a substrate to be taken into account, so that when patterns are applied to the substrate they may be applied in a desired (e.g. uniform) manner. The processing of a substrate may include, for example, providing an amount of material on the substrate (e.g. providing a layer of material on the substrate), or removing an amount of material from the substrate (for example, by the use of a developer or an etching process or the like), or the lithographic application of patterns to the substrate. It is usual for a model of a lithographic process to have built into it details of relationships between different processes that may be applied to the substrate and the affect of these processes on one another. For instance, for a pattern feature having a certain critical dimension or sidewall angle to be applied to a target portion of a substrate, a model of a lithographic process may be used to provide at least an indication of, or calculate, the required dose of radiation of a beam of radiation used to provide that pattern feature, or a focal property of that beam. In order to achieve this, the model of the lithographic process may need to have built into it relationships relating to how the provision of material on the substrate, or the removal of material from the substrate may affect the critical dimension or sidewall angle of the pattern feature for a beam of radiation providing a given dose of radiation and having a specific focal property.
In order to create and establish a model of a lithographic process, measurements may be taken. Relationships may then be determined based on the results of those measurements. Those relationships may then form the basis of the model, and allow the prediction or calculation of, for example, changes in the critical dimension and/or sidewall angle of a pattern feature as a function of process conditions. For instance, the thickness of resist deposited on a substrate may be measured. The critical dimension or sidewall angle of pattern features provided using that resist may be subsequently measured. A relationship between the critical dimension, sidewall angle and resist thickness can then be established and form the basis of at least a part of a model of a lithographic process. In future, this relationship can be used, for example, to control the dose of radiation provided by a beam of radiation in order to ensure that the critical dimension and sidewall angle of pattern features is more uniform across the substrate than if the variation in resist thickness had not been taken into account.
Resist (or any other material) that is provided on a substrate is not often provided with a uniform thickness, despite best efforts to avoid thickness variation. For instance, the deposition of resist or the like may have a radial thickness profile, for example, the resist has increased thickness from the centre of the substrate to the outside of the substrate, or decreased thickness from the centre of the substrate to the outside of the substrate. Such variation may be a result of spin-coating of the resist onto the substrate. Alternatively or additionally, a degree of removal of material from the substrate may also vary across the substrate. For instance, the etching of material from a substrate may result in more material being removed from one area of a substrate than from another area of the substrate. In summary, the processing of a substrate will have an associated fingerprint. This fingerprint may be, for example, a function of thickness of material provided on the substrate, or be a function of the amount of material removed from the substrate (e.g. a thickness variation profile, or an etch rate profile), or a combination of these functions. It is known to determine information regarding such a fingerprint and to form a model of a lithographic process that takes into account the fingerprint. Using such a model may thus ensure that pattern features may be applied to a substrate more uniformly.
Various different methods have been proposed for obtaining information for modeling a lithographic process, so that for example the fingerprint referred to above can be taken into account. Some of these methods include taking measurements of pattern features from only a small area of a substrate, which does not allow information across an entire or a majority surface of the substrate to be obtained. Other methods include using small target portions on a substrate, and varying exposure conditions between each target portion. However, while this method may allow some information to be determined across the entire or a majority surface of the substrate, it would be desirable to obtain even more information.
What is needed is to provide, for example, a method of obtaining information for use in modeling a lithographic process, and a lithographic arrangement for carrying out that method, which obviates or mitigates one or more problems of the prior art, whether identified herein or elsewhere, or which provides an alternative to an existing method or apparatus.